Portrait of Cosimo I de'Medici in Armor.

Cosimo I de’Medici was born in the city of Florence on June 12, 1519, into the family of a wealthy mercenary captain (or condottiere), Giovanni dale Bande Nere and his wife, Maria Salviati.

He first came to power in 1537 at the age of 17, succeeding Alessandro de’Medici following his assassination, due to Alessandro’s own son, Giulio, being too young at the time of his father’s death. Although Cosimo was little known around Florence at the time, many of the city’s officials and nobility supported his rise to power, hoping to install him as a puppet duke through whom to rule Florence themselves. However, Cosimo turned out to be far more ambitious and shrewd than they had hoped, and soon shrugged off their attempts to make him their subordinate, rejecting a clause he had signed which bestowed much of the power to a council of noblemen.

Almost immediately after his ascension to dukedom, Cosimo faced his first challenge in the form of many political exiles from Florence, among them the influential family of Strozzi, who jumped at the opportunity to seize the city after the death of Alessandro. The exiles joined forces and marched upon Tuscany under the command of Filippo and Piero Strozzi, where they were engaged by an army consisting of Cosimo’s finest troops, lead by Alessandro Vitelli, in the Battle of Montemurlo, just eighteen miles north of Florence. Cosimo’s forces successfully routed the enemy and pursue Filippo Strozzi and a number of his commanders into the Montemurlo fortress, managing to overwhelm the defenders within a few hours, and securing Cosimo’s hold on Florence.

Cosimo wasted no time in further consolidating his power, and the following month made a deal with Holy Roman Emperor Charles V to recognize him as the official head of Florence in exchange for military aid in the war against France. Two years later he married Eleanor of Toledo, a daughter of the Spanish Viceroy of Naples, Pedro Álvarez de Toledo, thus securing an important political link to Spain, which at the time had control over most of southern Italy, including Florence. As a result of both of these actions, many Imperial and Spanish garrisons were withdrawn from Tuscany, furthering Florence’s independence and the Medici’s power.

In 1554, the nearby city of Siena, an important contender for influence over Tuscany, rebelled against Charles V and expelled the Imperial garrison stationed there, siding with France in the ongoing war. Seizing the opportunity, Cosimo enlisted the help of the emperor in defeating the Sienese at the Battle of Marciano and besieged the city, finally taking it after over a year of resistance. By 1559, Cosimo annexed the last remaining shards of the Republic of Siena, thus establishing the Grand Duchy of Tuscany.